Microstructure evolution and mechanical properties analysis in dissimilar ultrasonic metal welding of aluminum to copper

Ultrasonic metal welding is a solid-state welding process being widely employed for joining the interconnections of lithium-ion batteries in hybrid and plug-in hybrid vehicles. Examining the microstructure of the joint is of crucial importance to evaluate the performance. This analysis allows a comprehensive understanding of the phenomena influencing the joint strength. In this study, ultrasonic welding was utilized to join copper to aluminum sheets of 0.3 mm thickness, and then the macro- and microstructural characteristics of the weld crosssection and interface were thoroughly investigated. Finally, the microstructural changes at different values of ultrasonic welding parameters were correlated with the mechanical strength characteristics. This research identified mechanical interlocking and interface diffusion as the primary welding mechanisms. With an increase in welding time from 0.2 to 0.8 s, the bond density rose, the post-weld thickness decreased, and the average Vickers hardness at various points along the weld interface decreased by 10%.